This invention relates to a process for preparing polyisocyanates such as are useful, for example, in making polyurethane polymers.
Polyisocyanates are commonly used in various applications, the most common of which is the preparation of polyurethane polymers. The most commercially important of these are aromatic polyisocyanates such as toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI) and the so-called polymeric MDI products which are usually formed as by-products in MDI production. However, certain aliphatic polyisocyanates are also used commercially, such as hexamethylene diisocyanate, isophorone diisocyanate and hydrogenated MDI (H.sub.12 MDI).
On a commercial scale, polyisocyanates are almost universally produced by reacting the corresponding polyamine with phosgene. For example, TDI is produced by reacting phosgene with toluene diamine. Several problems are associated with this process, the most significant of which is that large quantities of phosgene must be handled. Phosgene is highly toxic, and its handling requires special care to prevent exposure to workers. HCl is produced in the reaction of the phosgene and the polyamine, and must be neutralized or otherwise removed. The phosgene and HCl also engage in various side reactions, producing unwanted chlorinated by-products which affect the quality and purity of the product polyisocyanates. Thus, a process by which polyisocyanates can be prepared without using phosgene would be highly desirable.
Several non-phosgene routes to polyisocyanates have been developed. One such route involves the preparation of an n-alkyl biscarbamate, which is subsequently pyrolyzed to form the corresponding polyisocyanate and alcohol. The biscarbamate is generally prepared by one of two methods. The first such method involves the oxidative carbonylation of an amine with carbon monoxide, oxygen and an alcohol such as ethanol or methanol, to form the corresponding ethyl- or methylcarbamate. The other method involves a reductive carbonylation of a nitro compound with carbon monoxide and an alcohol such as ethanol or methanol to form the corresponding ethyl- or methylcarbamate. See, e.g., WO 86-05179.
In these processes, the ethyl- or methylpolycarbamates are pyrolyzed at high temperatures and under reduced pressures. At these temperatures, the polyisocyanates often polymerize as they form, producing tars and other by-products. For this reason, and because these carbamates are often crystalline, a solvent is required as a diluent. The use of a solvent in the pyrolysis adds costs in recovering the product polyisocyanate from the solvent. To a lesser extent, the high temperature required further increases the cost of these processes.
Thus, it would be desirable to provide a route to make polyisocyanates through a polycarbamate intermediate, wherein the pyrolysis of the polycarbamate can be conducted in the absence of solvent, and preferably at moderate temperatures.